(i) Field of the Invention
The instant invention relates to a novel method of measuring fibre properties, particularly residual lignin content of chemical pulp, with the aid of a fast scanning spectrometer in the combined visible and near infrared spectral regions.
(ii) Description of the Prior Art
The accurate on-line measurement of the Kappa number of chemical pulps obtained from rapidly varying furnishes remains an unresolved issue for chemical pulp manufacturers. Mills that use residual sawmill-chips from various locations and those producing specialty grades are especially affected because of the variability of chip quality from various suppliers. This issue is even more prevalent now due to a shortage of available market wood chips and the fact that pulp and paper mills are being forced to purchase chips geographically distant from their manufacturing facilities. All too often, pulp produced during grade changes, or from poorly characterized chip species mixtures, has to be downgraded because of a high proportion of improperly cooked pulp, since each wood chip species cooks differently. Analysers capable of rapid determination of Kappa number could help mills greatly reduce Kappa number variation, bleaching costs and the amount of off-grade pulp.
Current digester control algorithms use the blow-line Kappa number in conjunction with the H-factor [1] for controlling the pulping operation. Generally, Kappa number measurements are required as a feedback parameter to allow for adjustments in the liquor charged to the digester at the various zones of the cook and, much more critically, as a feed-forward parameter for control of the bleach plant. Higher Kappa number pulp requires higher charges of bleaching chemical to reach target final pulp brightness levels, particularly during the oxygen-delignification stage.
The measurement of residual lignin content in pulp has been traditionally done on an hourly basis as a laboratory analysis according to TAPPI standard method T236 [2] which uses a back titration of residual permanganate with potassium iodide. However, the method requires extensive workup and can take 30 to 60 minutes per sample. Jiang et al. [3] have improved on this standard technique by semi-automating the titration process with an automatic, multi-sample titrator. More recently, Chai et al. [4] have proposed the use of rapid acidification to improve the accuracy of the potassium permanganate titration. Manganese dioxide precipitation is prevented and thus residual permanganate can be analysed without spectral interference from MnO2 and allowing the UV-visible spectrometry technique to be more accurate than by titration. However, this method still requires sample preparation, a number of reagents and a chemical reaction which takes between three to five minutes to complete. The time delay limits the utility of this method for feedback control of the digester and for feed-forward control of the bleach plant.
Current commercially available Kappa number analysers use UV light with a combination of reflectance, scattering, transmittance, and consistency measurements [5-6] to analyse pulp samples with frequencies in the order of 10 to 20 minutes. These methods (STFI, Metso, and BTG) utilize a transparent cell/chamber through which a thoroughly washed pulp, diluted to a known consistency (0.1% to 0.4%) is circulated, whereby the reflected and transmitted light is collected at some predetermined UV or visible wavelengths over a period of one or more minutes, and a consistency-corrected Kappa number is determined from these readings so as to account for the change in reflected intensity which is strongly dependent on consistency. The UV-sensor is used to measure residual lignin while the visible light is used for consistency measurement. A typical routine requires extensive washing to remove excessive residual liquor. Dilution is then carried out to approximate volume and the pulp slurry is circulated and a separate detector is used for consistency determination. If the consistency is not within the desired range, the dilution is adjusted and the pulp mixture is then again re-measured. Upon reaching the desired consistency, Kappa measurements are made. Although the principle is simple, the actual measurement is complex because lignin absorption cannot be measured accurately without accounting for the interferences produced by changes in pulp consistency and furnish. This problem can be addressed by building two-point calibrations that are valid for a relatively narrow range of sampling conditions. Calibrations are prepared by characterising the relationship between the three types of measurement at a given optimal consistency, and are reported to be satisfactory for bleach-plant samples [6], single-species furnishes and stable, well characterised mixed furnishes of constant composition.
Currently available commercial kappa analysers do not provide accurate results for furnishes of unknown or rapidly changing composition [7]. When the composition of chips is constantly changing, instruments have to be constantly re-calibrated to follow the changes in furnish. Updating the two-point calibration and the sampling system requires constant attention from instrumentation personnel. Furthermore, owing to the added sample preparation step, throughput is relatively low, allowing throughput of only about two samples per hour for each location.
Lignin chemists have been using vibrational spectroscopy for nearly fifty years to characterise wood and pulp samples. Marton and Sparks [8] have determined the Kappa number of pulps by using the area beneath the lignin peak at 1510 cm−1 and the cellulose peak at 1100 cm−1 as an internal standard. The lignin/cellulose peak-area ratio was found to be insensitive to variations in basis weight. Similarly, Berben et al. [9] developed a method using infrared diffuse reflectance for estimating lignin content in unbleached pulp. A linear relationship for all species combined is found between the area of the band at 1510 cm−1 and Kappa number for a wide variety of species. However, these methods used dry pulp samples and are not amenable to online process analysis of Kappa number for process control.
U.S. Pat. No. 4,743,339 [10] illustrates a method for determining pulp properties, including Kappa number using FT-IR in the spectral range of 6300 nm to 7800 nm. In this method, a spectrum, acquired with 200 co-add averages, needs to be baseline corrected by first determining the water content and fibre content (consistency). This method is extremely sensitive to consistency since it must be determined so as to provide an accurate baseline correction. Another short-coming of this method is that it is sensitive to species and must be recalibrated with changes in digester furnish. Furthermore, the number of scans limits this measurement technique as an online analyser since it takes over 15 minutes for each spectral acquisition, not including the sample preparation time, and must be performed at room temperature.
Yuzak and Lohrke [11] detailed the results of a series of experiments and showed that NIR can be used to estimate the Kappa number of properly prepared kraft pulp samples, i.e. dried handsheets, with an error of ±2.0 Kappa. The authors concluded that, through their series of sample pre-treatment methods, NIR spectral model for the determination of Kappa number are: 1. Without pre-treatment—unacceptable, 2. Hose-washed—unacceptable (error −9.0 to +11.7), 3. Hose-washed and filtered—unacceptable (error −11.8 to +4.3), 4. Hosed-washed+blended+filtered+pressed—unacceptable (error −0.3 to −15.2), 5. Hose-washed+blended+dried (handsheet)—acceptable (error ±2.0 Kappa). Even though the authors utilized the spectral region of 1500-1750 nm and 2100-2400 nm, their reliance on homogenizing and drying the samples effectively teaches away from using NIR spectrometry as a rapid on-line method for determining Kappa numbers.
U.S. Pat. No. 5,536,942 [12] describes a method and apparatus for the measurement of properties, including Kappa number, of fibres in a fibre suspension with the aid of an NIR spectrometer. The invention details the steps and apparatus for extracting the samples from the process stream, repeated washing in a chamber, and pumping the diluted solution to a cell which incorporates a screen whereby the fibres are concentrated and monitored at 950 nm to an absorbance of 2.0 to 4.5 absorbance unit (A.U.) to obtain the preferred consistency (3%), and registering with the detector to obtain a transmission NIR spectrum in the range of 850 nm to 1050 nm. The sample is then re-homogenized by backflushing the cell and re-concentrating the fibres on the screens then repeating the acquisition. This method also heavily relies on the measurement of consistency and operates at high absorbance range, outside the typical linear Beer-Lambert's law and reaching the limit of linear range of many instruments. As a result, for the range of consistencies used by U.S. Pat. No. 5,536,942, slight errors in the absorbance would translate into large errors associated with Kappa-number determination. In addition, the requirement of extensive washing and concentrating prior to spectral data acquisition then followed by re-homogenizing and concentrating and data acquisition also limits the true online feasibility of the measurement technique for process control.
PCT Patent WO 01/79816 [13] describes a method for the determination of physical properties of fibre suspensions such as viscosity, tensile strength, fibre lengths, density, burst index, coarseness, opacity, beating requirement, light scattering, zero span as well as chemical compositions such as lignin and hexanuronic acid. The sample is withdrawn from the process and is washed to provide clean pulp which is diluted to two streams with one partial flow to be dewatered and dried and used for spectroscopic analysis while the second partial flow is used for analysis of physical fibre data by means of image analysis. The two data sets are combined with multivariate data processing for predictions of physical fibre properties. The method states that the correlation is improved with the combination of data from the FibreMaster and NIR data. Spectroscopic measurement is made in the NIR range from 780 nm to 2500 nm. The diluted sample needs to be dried to a solids content of at least 50%, preferably 70%, which is accomplished by filtering and forced air drying preferably by means of direct contact with compressed air. The method further states that the drying process takes time, but the image analysis also takes time and this allows for the synchronization between the two techniques. As such, the throughput of the stated method can only reach four analyses per hour, and as described is unsuitable for an on-line application. Also, no data for Kappa number was presented.
Birkett and Gambino [14] further details the result as obtained with a filtometer or filter-based spectrometer and showed correlations for handsheets kappa number made from Eucalyptus grandis and 5 specific wavelengths that have been optimized by multilinear regression. The author showed that there is species dependency associated with filter-based spectrometers as models developed for E. grandis were not able to provide acceptable results as determined by the overall calibration model (p. 195, 1st par., line 5 to 17). The author further states that, (p. 195, par. 3, lines 1-3) “ . . . that calibrations for pine and eucalyptus should be treated separately” and that “ . . . it may be necessary to calibrate for a specific species . . . ”. Furthermore, the author is providing results done with dried handsheets of pulp. Birkett and Gambino acknowledges this particular matter by stating that the results are obtained with dried handsheets and that “ . . . the ability to use NIRS on wet pulp obviously would make process control easier and faster” (p. 196, par. 1, lines 8-13). Birkett and Gambino showed that filtered-based NIR system is sensitive to species variation and can only be applied to dried handsheets.
U.S. Pat. No. 5,953,11 [15], Millar et. al. describes the use of a continuous in-line kappa measurement system whereby light from an excitation source is injected into a flowing conduit carrying pulp. Reflected light is collected with two detectors, one at near-proximity and one at far-proximity along with a light-source feed back as a reference. The reflected light collected at the near-proximity and far-proximity detectors are normalized with the reference and used for calculations of kappa number. The illumination light is made up of individual specific wavelengths in the visible spectral consisting of a wavelength in the blue region, green region, amber, and red region (page 6, paragraph 4, line 7-10). As with many other systems currently available, this system mainly relies on the lignin absorbance in the visible region of single wavelength, as in a filtometer or filter-based visible spectrometer. Though filter-based systems are relatively inexpensive and can be configured with many different wavelengths, filter-based system suffer from wavelength accuracy from filter-to-filter due to manufacturing processes as well as calibration drifts and the extensive calibration requirements due to system to system differences. Furthermore, Birkett and Gambino [14], above, showed that NIR filter-based system can not handle species variation and requires dried handsheets in order to provide acceptable kappa number for process control. Due to these shortcomings and difficulties, filter-based systems are generally not successful as online analysers.
Poke et al., [16] present a NIR method for the determination of lignin in wood meal, which requires the drying and grinding of samples. Again, this method is clearly unsuitable for an-on-line application.
To overcome the limitations of NIR spectrometry, Trung et al. [17] have proposed the use of visible-excitation Raman spectrometry for measuring lignin in pulp. Even though this method overcomes some of the limitations associated with laser-induced fluorescence, this method requires the preparation of a high-consistency sample (15 to 30%) and a relatively long acquisition time (5 to 10 minutes), primarily because of the inherent weakness of the Raman signal produced by the small illumination spot used in the application. The small illumination spot limits the amount of pulp being sampled through poor sub-sampling, thereby increasing the likelihood of getting a non-representative sample for analysis. This will increase the uncertainty of the measurement since Kappa number is known to vary significantly from fibre-to-fibre, within a cook.
Therefore, the prior art clearly teaches away from the use of NIR spectrometry for determining Kappa number on wet pulp samples, especially if one wishes to perform any rapid, on-line quantitative analysis. Unexpectedly, the instant invention provides a very rapid method for the quantitative determination of lignin content or Kappa number in wet pulp samples. As NIR spectrometry is repeatedly described in the prior art as being quite sensitive to moisture content, this further teaches those skilled in the art of pulp analysis, away from applying NIR spectrometry for the measurement of lignin content or Kappa number. None of the methods cited in the prior art is capable of determining lignin content with sufficient accuracy and detail to yield a useful measurement for process and/or quality control. In the following, we disclose such a method. The instant invention overcomes the limitations described above by performing measurements on a large amount of pulp, and, unlike the prior art, can also tolerate moderate variations in consistency.